(a) Field of the Invention
The present invention relates to a lead frame used as a substrate of packages for mounting semiconductor elements. More specifically, it relates to a lead frame for use in a leadless package such as a quad flat nonleaded package (QFN), a small outline non-leaded package (SON), or the like, which is molded with resin by a mass molding in an assembly process of the packages, also to a method of manufacturing the lead frame and to a method of manufacturing a semiconductor device using the lead frame.
(b) Description of the Related Art
When a leadless package such as a QFN is fabricated, a basic process thereof includes the steps of mounting semiconductor elements severally on die-pad portions of a lead frame (die bonding), electrically connecting electrodes of the semiconductor elements to lead portions of the lead frame with bonding wires (wire bonding), molding the semiconductor elements, the bonding wires and the like with molding resin (molding), dividing the lead frame into individual semiconductor devices (dicing), and the like.
Also, as the type of molding, there are an individual molding method in which the semiconductor elements are individually molded with resin, and a mass molding method in which a plurality of semiconductor elements are severally molded together with resin. However, since a resin molding is individually performed for each of the semiconductor elements in the individual molding method, there is a difficulty in terms of an efficient package assembly as compared to the mass molding method. Accordingly, the mass molding method has been a mainstream in recent trends.
In the prior art, it is a common practice to perform such a mass molding in the state in which an adhesive tape is attached to an entire back surface of the frame, so as to prevent a leakage of molding resin (which is also referred to as “mold flush”) from the back surface of the frame. Namely, an attaching of the adhesive tape to one of the surfaces of the lead frame (taping) has generally been performed prior to the package assembly process.
Use of the mass molding method is advantageous in terms of an efficient package assembly. However, since the adhesive tape is attached to one of the surfaces of the lead frame as mentioned above, a problem occurs in the event of a heat treatment in the molding step in that the lead frame is warped due to a difference in the coefficient of thermal expansion between a material (metal) constituting the lead frame and a material (organic resin or the like) constituting the adhesive tape. Presence of such a warp inhibits a stable package assembly and thereby causes a lowering in reliability as semiconductor devices.
In view of this, as an approach to deal with the disadvantage, a method of attaching adhesive tapes which are sectioned for respective regions on a lead frame required upon resin molding has been adapted instead of attaching an adhesive tape entirely on one of the surfaces of the lead frame. By sectioning the adhesive tape into tiny pieces and attaching them, it is possible to reduce a shrinkage of the individually sectioned tapes and to thereby reduce a warp of the entire lead frame.
However, it is necessary to increase the frequency of the taping step in accordance with the number of the sectioned adhesive tapes, which has been sufficient just by once (namely, the load on the taping step is increased). As a result, there has been a problem of a lowering in efficiency in the process of manufacturing a lead frame.